1. Field of the Invention
The present invention generally relates to systems for control of a gas appliance and more particularly relates to electronic control of an auxiliary electric motor.
2. Description of the prior art
It is known in the art to employ various appliances for household and industrial applications which utilize a fuel such as natural gas (i.e., methane), propane, or similar gaseous hydrocarbons. Typically, such appliances have the primary heat supplied by a main burner with a substantial pressurized gas input regulated via a main valve. Ordinarily, the main burner consumes so much fuel and generates so much heat that the main burner is ignited only as necessary. At other times (e.g., the appliance is not used, etc.), the main valve is closed extinguishing the main bumer flame.
A customary approach to reigniting the main burner whenever needed is through the use of a pilot light. The pilot light is a second, much smaller burner, having a small pressurized gas input regulated via a pilot valve. In most installations, the pilot light is intended to burn perpetually. Thus, turning the main valve on provides fuel to the main burner which is quickly ignited by the pilot light flame. Turning the main valve off, extinguishes the main burner, which can readily be reignited by the presence of the pilot light.
These fuels, being toxic and highly flammable, are particularly dangerous in a gaseous state if released into the ambient. Therefore, it is customary to provide certain safety features for ensuring that the pilot valve and main valve are never open when a flame is not present preventing release of the fuel into the atmosphere. A standard approach uses a thermogenerative electrical device (e.g., thermocouple, thermopile, etc.) in close proximity to the properly operating flame. Whenever the corresponding flame is present, the thermocouple generates a current. A solenoid operated portion of the pilot valve and the main valve require the presence of a current from the thermocouple to maintain the corresponding valve in the open position. Therefore, if no flame is present and the thermocouple(s) is cold and not generating current, neither the pilot valve nor the main valve will release any fuel.
In practice, the pilot light is ignited infrequently such as at installation, loss of fuel supply, etc. Ignition is accomplished by manually overriding the safety feature and holding the pilot valve open while the pilot light is lit using a match or piezo igniter. The manual override is held until the heat from the pilot flame is sufficient to cause the thermocouple to generate enough current to hold the safety solenoid. The pilot valve remains open as long as the thermocouple continues to generate sufficient current to actuate the pilot valve solenoid.
The safety thermocouple(s) can be replaced with a thermopile(s) for generation of additional electrical power. This additional power may be desired for operating various control circuitry of equipment auxiliary to the gas appliance. Normally, this requires conversion of the electrical energy produced by the thermopile to a voltage usefull to these additional loads. Though not suitable for this application, U.S. Pat. No. 5,822,200, issued to Stasz; U.S. Pat. No. 5,804,950, issued to Hwang et al.; U.S. Pat. No. 5,381,298, issued to Shaw et al.; U.S. Pat. No. 4,014,165, issued to Barton; and U.S. Pat. No. 3,992,585, issued to Turner et al. all discuss some form of voltage conversion.
For gas appliances intended to provide space heating, it is customary to employ auxiliary circulating components, such as electric motor powered fans. Typically, such auxiliary circulating components are controlled using manually operated switches.
The present invention overcomes the disadvantages of the prior art by providing a totally electronic technique for the control of an auxiliary circulating fan within a gas appliance. In the preferred mode of the present invention, the gas appliance is a gas fireplace. Room air is circulated through the heat exchanger of the gas fireplace by an electric motor powered air circulating fan. Though the fan is powered by the normal alternating current found in the home, the control circuits are all powered by heat from the pilot light flame. The rotational speed of the electric motor, and therefore the amount of air circulation, is continuously selectable by the control circuitry.
In accordance with the preferred mode of the present invention, a thermopile is thermally coupled to the pilot flame. As current is generated by the thermopile, it is converted via a DC-to-DC converter to a regulated output and an unregulated output. The regulated output powers a microprocessor and other electronic circuitry which control operation of the main fuel valve, remote communication with the operator, and speed of the circulating fan. The unregulated output powers various mechanical components including a stepper motor which controls the main burner valve.
The circulating fan electric motor is powered by the 110 volt a.c. line. The circuitry of the present invention regulates the effect duty cycle of this line input to regulate the rotational speed. A phase detector senses the line input and notifies the microprocessor of the timing of the zero crossing of the 110 volt signal. The microprocessor turns a triac to the on state to power the electric motor after a suitable delay period. Increasing the delay period decreases the effective duty cycle and therefore slows the operational speed. Decreasing the delay period, increases the speed.
Because the microprocessor operates in digitally clocked sequences, the speed variations are actually made in discrete steps, even though these discrete steps are so small as to make the speed variations seem continuous. However, the present invention can also be implemented using analog control, if completely continuous speed control is required. In the preferred system, six individual discrete steps are used to simplify the operator interface.